This project will deal with a physicochemical investigation and comparison of the properties of the cytoplasmic and mitochondrial forms of the enzyme malate dehydrogenase isolated from porcine heart. Recent work by this laboratory has implicated the essentiality of four residues in the active center of the mitochondrial enzyme, i.e. histidine, cysteine, lysine and arginine. In addition, the work by this laboratory has implicated the essentiality of an arginine and a lysine residue in the active center of the cytoplasmic form of the enzyme. Sequence studies are being pursued on labeled peptides containing these residues in order to identify other residues in their proiximity. Bifunctional reagents will be utilized in order to investigate the proximity of each of these active center residues to one another. The objective of this work will be to attempt to understand the function of each of these residues in the mechanism of action of these enzymes. Additional work related to the subunit structure of these enzymes and the observed pH dependent dissociation of these subunits is currently under investigation in an attempt to understand the role of subunit dissociation in normal enzymes catalysis. BIBLIOGRAPHIC REFERENCES: Inactivation of Porcine Heart Cytoplasmic Malate Dehydrogenase by Pyridoxal-5'-Phosphate. D.M. Bleile, J.L. Jameson, and J.H. Harrison, The Journal of Biological Chemistry, 251 pp. 6304-6307, 1976. Investigation of the Subunit Interactions in Malate Dehydrogenase. D.M. Bleile, R.A. Schulz, and J.H. Harrison, The Journal of Biological Chemistry. Vol. 252, pp. 755-758, 1977.